For purposes of the present discussion, a focus sensing system is a device that senses focus for, and enables control of, a focusing optical device. Because the usable depth of field of an imaging device decreases rapidly with the increase in resolution, focus control is especially critical in microscopy. In its most general form an ideal focus sensing device optically senses a wavefront curvature term that indicates the state of focus of the wavefront, while being insensitive to other kinds of wavefront shapes. Such a system may be a component of a focus controlling servomechanism, i.e., an autofocus system, or it may be used to measure the distance between a reference surface and a target surface without controlling focus.
The state of focus of an optical system can be described as the spherical curvature of an idealized electromagnetic wavefront as it propagates through the system. An ideal focus sensing method should reject wavefront information other than that which indicates the state of focus. In short, focus is a phase-related term of an idealized wavefront as it propagates through an optical system at the speed of light. However, proper sensing of focus is complicated by the fact that available square-law detectors are sensitive to the square of the amplitude of the wave, but are not directly sensitive to the complex (in the mathematical sense of real plus imaginary) properties that describe a wave, namely phase or amplitude. A robust focus sensor must also separate the focus signal from other variables, including those that are related to the target as well as those emanating from the focus sensing system itself.
In addition to square-law detectors, other optical components have been used for the task at hand. These include, among others, lenses, mirrors, beam splitters, and various filters for the refraction and reflection of light, for wavelength selection or rejection, for manipulation of polarization, and other typical optical purposes. When optical filters have been used they have been designed to operate as two-dimensional devices. That is, their salient properties are intended to operate on the two-dimensional properties of a beam of light as it passes through the filters. Even though a filter may have been composed of three-dimensional structures, its function has been essentially restricted to two-dimensions with regard to the light beam itself. In fact, in many cases those filters may be translated axially by a relatively large distance as compared to the depth of focus of image-forming components without significantly altering their function in the system. By contrast, the specially constructed filter used in the optical system described here improves both the axial and radial focus-sensing operation of square-law detectors. These filters not only employ three-dimensional structures, but also function fully in three dimensions by means of refraction, diffraction, interference or scattering. The functions of the described filters require unique axial and radial positioning in order to achieve optimal performance. The functionality of previous filters has been degraded by sensitivity to errors caused by sample topography, tilt, reflectance variation and surface patterns; furthermore, non-ideal properties of the measurement device itself have caused significant errors.